Long Term Evolution (LTE) networks are designed to support user equipment (UEs) from different 3rd Generation Partnership Project (3GPP) releases (e.g., LTE Rel-8/9/10/11) in a backward compatible way. One of the LTE network design objectives is to enable co-scheduling of such UEs in time, frequency and space dimensions with as few scheduling constraints as possible. This may include, for example, multi-user Multiple-Input-Multiple-Output (MU-MIMO).
Furthermore, the LTE standard should be able to support various and flexible deployments. Some examples of expected deployments for modern LTE networks (Rel-11 and beyond) include, e.g.:                Macro-deployments, where large cells are typically divided into independent sectors;        HetNet-deployments, where pico-cells are deployed within the coverage of a macro-cell in order, e.g., to improve coverage for high data rate UEs; and        Hotspot scenarios where an access point serves a small area with high throughput need.        
A “cell” is characterized in LTE by a “cell-ID” and a carrier frequency, which affects several cell-specific algorithms and procedures.
In addition, LTE networks are designed with the aim of enabling optional CoMP (Coordinated multipoint processing) techniques, where different sectors and/or cells operate in a coordinated way in terms of, e.g., scheduling and/or processing. An example is uplink (UL) CoMP where a signal originating from a single UE is typically received at multiple reception points and is jointly processed in order to improve link quality. UL joint processing (also referred to as UL CoMP) allows transformation of what is regarded as inter-cell interference in a traditional deployment into a useful signal. Therefore, LTE networks taking advantage of UL CoMP may be deployed with a smaller cell size compared to traditional deployments in order to fully take advantage of the CoMP gains.
LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in the downlink and DFT-spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, with each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms (see FIG. 2). Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RBs), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is being transmitted to and upon which resource blocks the data is being transmitted in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3, or 4 OFDM symbols in each subframe. A downlink system with 3 OFDM symbols as control signaling is illustrated in FIGS. 1-3.
LTE uses Hybrid Automatic Repeat Requests (HARQ), where, after receiving downlink data in a subframe, a UE attempts to decode it and reports to the base station whether the decoding was successful (ACK) or not (NAK). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data. Uplink control signaling from the terminal to the base station for HARQ consists of:                HARQ acknowledgements for received downlink data;        terminal reports related to the downlink channel conditions, used as assistance for the downlink scheduling; and        scheduling requests indicating that a mobile terminal needs uplink resources for uplink data transmissions.        
In order to achieve a convenient trade-off between scheduling flexibility and compact signal dynamic range (or, equivalently, low cubic metric (CM)), DFT-spread OFDM (DFTS-OFDM) is employed in LTE UL. DFTS-OFDM, also known as “single-carrier OFDM,” is a modulation technique where the OFDM IDFT modulator is preceded (at the transmitter) by a DFT precoder operating over the scheduled bandwidth. The assigned bandwidth needs to be contiguous, i.e., no sparse allocation of RBs is supported in the UL. At the receiver, the DFT OFDM demodulator is followed by an IDFT decoder.
In LTE, multi-cluster OFDM is also optionally supported in the UL, where two non-contiguous clusters of RBs are associated in the same subframe, thus violating the single carrier property of DFTS-OFDM. A single DFT precoder is employed, and the precoded symbols are mapped to two clusters of non-contiguous subcarriers.
If a mobile terminal has not been assigned an uplink resource for data transmission, the L1/L2 control information (e.g., channel-status reports, hybrid-ARQ acknowledgments, and scheduling requests) is transmitted in uplink resources (resource blocks) specifically assigned for uplink L1/L2 control on the Physical Uplink Control Channel (PUCCH). As illustrated in FIGS. 1-4, these resources are located at the edges of the total available cell bandwidth. Each such resource consists of twelve “subcarriers” (one resource block) within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources utilize frequency hopping on the slot boundary, e.g., one “resource” consists of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe or vice versa. If more resources are needed for the uplink L1/L2 control signaling, e.g., in case of very large overall transmission bandwidth supporting a large number of users, additional resource blocks can be assigned next to the previously assigned resource blocks.
The reasons for locating the PUCCH resources at the edges of the overall available spectrum are two-fold:                together with the frequency hopping described above, this maximizes the frequency diversity experienced by the control signaling; and        assigning uplink resources for the PUCCH at other positions within the spectrum (i.e., not at the edges), would have fragmented the uplink spectrum, making it impossible to assign very wide transmission bandwidths to a single mobile terminal and still retain the single-carrier property of the uplink transmission.        
The bandwidth of one resource block during one subframe is too large for the control signaling needs of a single terminal. Therefore, to efficiently exploit the resources set aside for control signaling, multiple terminals can share the same resource block. This is done by assigning the different terminals different orthogonal phase rotations of a cell-specific length-12 frequency-domain sequence. A linear phase rotation in the frequency domain is equivalent to applying a cyclic shift (CS) in the time domain. Thus, although the term “phase rotation” is used herein, the term cyclic shift is sometimes used with an implicit reference to the time domain. FIG. 4 illustrates an example phase shift of an uplink L1/L2 control signaling transmission on the PUCCH.
The resource used by the PUCCH is therefore not only specified in the time-frequency domain by the resource block pair, but also by the phase rotation applied. Similarly to the case of reference signals, there are up to twelve different phase rotations specified, providing up to twelve different orthogonal sequences from each cell-specific sequence. However, in the case of frequency-selective channels, not all the twelve phase rotations can be used if orthogonality is to be retained. Typically, up to six rotations are considered usable in a cell.
As mentioned above, uplink L1/L2 control signaling include hybrid-ARQ acknowledgements, channel-status reports and scheduling requests. Different combinations of these types of messages are possible as described further below, but to explain the structure for these cases it is beneficial to discuss separate transmission of each of the types first, starting with the hybrid-ARQ and the scheduling request. There are two formats defined for the PUCCH, each capable of carrying a different number of bits.
Resource Block Mapping for PUCCH
The signals described above, for both of the PUCCH formats, are, as already explained, transmitted on a resource block pair with one resource block in each slot. The resource block pair to use is determined from the PUCCH resource index. Thus, the resource block number to use in the first and second slot of a subframe can be expressed as:RBnumber(i)=f(PUCCH index,i)  eq. (1)
where i is the slot number (0 or 1) within the subframe and f is a function found in 3GPP specifications.
Multiple resource block pairs can be used to increase the control-signaling capacity. When one resource block pair is full the next PUCCH resource index is mapped to the next resource block pair in sequence. The mapping is in principle done such that PUCCH format 2 (channel-status reports) is transmitted closest to the edges of the uplink cell bandwidth with the semi-static part of PUCCH format 1 next and finally the dynamic part of PUCCH format 1 in the innermost part of the bandwidth.
Three semi-static parameters are used to determine the resources to use for the different PUCCH formats:                NRB(2), provided as part of the system information, controls on which resource block pair the mapping of PUCCH format 1 starts;        NPUCCH(1) controls the split between the semi-static and dynamic part of PUCCH format 1; and        X controls the mix of format 1 and format 2 in one resource block. In most cases, the configuration is done such that the two PUCCH formats are mapped to separate sets of resource blocks, but there is also a possibility to have the border between format 1 and 2 within a resource block.        
The PUCCH resource allocation in terms of resource blocks is illustrated in FIGS. 1-5. The numbers 0, 1, 2, . . . represent the order in which the resource blocks are allocated to PUCCH (i.e., a large PUCCH configuration may need resource 0-6 while a small configuration may use only 0).
The introduction of new deployments as described above and the extensive use of UL CoMP implies a certain degree of fragmentation in the use of UL resources, especially at band edges where PUCCH resources are not dynamically optimized at each subframe. As a consequence, some UL RBs may be available at both band edges at certain subframes.
One possibility would be to exploit such RBs by scheduling PUSCH transmissions over them. However, due to the limited size of such resource blocks, the transmission would be quite inefficient. Another possibility would be to exploit both unused band edges for the same UE by multi-cluster OFDM. However, such a technique is associated with increased CM and it is therefore supported by a minority of terminals, if any.